1. Field of the Invention
The present invention relates to a polishing apparatus for polishing a periphery, including a flat portion, of a substrate.
2. Description of the Related Art
In semiconductor device fabrication, a wafer is used as a substrate. Processes of fabricating the semiconductor device include sequential film forming processes and film removing processes. As the film forming processes and the film removing processes are repeated, a periphery of the wafer becomes rough. As a result, fine contaminants, which are called particles, are produced from the roughed surface.
If the contaminants are left on the substrate, part of the contaminants can move from the periphery to a central portion of a surface of the wafer through transferring and processing of the wafer. Devices are typically formed in a grid pattern on the surface of the wafer. If the contaminants are attached to the devices, quality of the devices is lowered, resulting in defective products. In order to avoid such problems, it is necessary to process the periphery of the wafer.
As described above, in the fabrication processes of the semiconductor device, multiple films are formed on the wafer. In the film forming processes, there is a need to remove only a periphery of a film formed on the wafer. For example, after a certain film is formed, the wafer is transferred to the subsequent process, with the periphery of the wafer being held by a transfer robot. During the transferring of the wafer, the film on the periphery of the wafer could adhere to the transfer robot. This film could spread as contaminants in the subsequent processes. To avoid such spread of the contaminants, the portion of the wafer where the transfer robot grabs (i.e., the periphery of the wafer) is processed in advance, so that the film is removed from the periphery of the wafer.
One of the methods of processing the periphery of the wafer is a polishing process. The polishing process is classified roughly into two types: a one-side polishing method and a both-side polishing method. The one-side polishing method is performed by bringing a polishing tool into contact with a substrate from one side of the substrate. On the other hand, the both-side polishing method is performed by bringing a polishing tool into contact with a substrate from both sides of the substrate. Japanese laid-open patent publication No. 2001-205549 discloses a one-side polishing method performed by bringing a polishing cloth into contact with a substrate from one side of the substrate. In this patent publication, the polishing cloth applies a downward force to the periphery of the substrate, while the polishing cloth is moved from the periphery to the center of the substrate. Japanese laid-open patent publication No. 2007-208161 discloses another one-side polishing method that is performed by applying a downward force from a polishing drum to an upper bevel of a substrate. Japanese laid-open patent publication No. 2005-305586 discloses still another one-side polishing method that is performed by pressing a polishing tape against a periphery of a substrate from above or below the substrate. In these three methods, the pressing force can bend the periphery of the substrate. As a result, it is difficult to polish the periphery while keeping the polishing tool parallel to a flat portion of the periphery of the substrate.
Japanese laid-open patent publications No. 2005-277050 and No. 2007-189208 disclose a both-side polishing method performed by bringing a polishing tool into contact with a wafer from both sides of the wafer. According to this polishing method, the wafer is not bent, because the polishing tool holds the wafer from both sides of the wafer. However, in these two methods, a polishing surface of the polishing tool and a surface of the wafer are not parallel to each other. As a result, it is difficult to uniformly polish the flat surface of the periphery of the wafer while keeping an angle of the flat surface.
Japanese laid-open patent publication No. 2004-241434 discloses a solution to the above-mentioned problems. This polishing method is performed by bringing a polishing tool into contact with a periphery of a wafer from right above and right below the wafer. This method does not bend the wafer and is therefore suitable for planarizing the periphery of the wafer. However, this polishing method inevitably processes both an upper periphery and a lower periphery of the wafer, and cannot satisfy the need to polishing only a periphery at one side of the wafer.